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larikang t1_j0h6gbc wrote

Reply to comment by midnight_mechanic in Does rotation break relativity? by starfyredragon

> Also with gravity we feel the force of gravity pushing us downward

Isn’t it the other way around in relativity? When you are in free fall, you feel no force from gravity. When you are standing still you feel the force of the ground accelerating upwards into you.

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midnight_mechanic t1_j0h9sxi wrote

No. That's all backwards.

In free fall you are accelerating towards the earth (or whatever massive object). As you get closer to the massive object your rate of acceleration will actually increase. The acceleration due to gravity is 10 or 12% less at the international space station than it is on earth, for example.

You can always measure when you are in a gravitational field (or being accelerated). For that reason, it wouldn't be correct to say you don't feel any force from gravity.

The force due to gravity you feel on earth is you are being pressed/accelerated into the surface of the earth. Whatever you are standing on is strong enough to resist the pressure from you being forced into it, unless the floor collapses.

The ground is *stationary. It doesn't accelerate into you.

Stationary is a relative term that means whatever I want it to mean as long as my frame of reference is consistent. For this reason, for these examples, the earth is unmoving, stationary and eternal.

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EuphonicSounds t1_j0kyyzz wrote

Actually, you have it backwards: an accelerometer in free-fall measures 0. If you're interested, read up on the equivalence principle, which is a cornerstone of general relativity.

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larikang t1_j0hdt9a wrote

> You can always measure when you are in a gravitational field (or being accelerated). For that reason, it wouldn't be correct to say you don't feel any force from gravity.

When astronauts do low-G training in the vomit comet they are definitely in a gravitational field and (in your words) are "being accelerated". But they feel no force of gravity.

Or as a similar thought experiment: imagine that I am in a spaceship with no windows traveling at a constant speed far from any large mass. If the spaceship started to approach a large planet, would I feel a force of gravity pulling me in toward the planet stronger and stronger as I approached it? My understanding of relativity is no: since I am in an inertial reference frame it makes no difference to me.

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